Conventional steel technology is based on manipulating a solid-state transformation called a eutectoid transformation. In this process, steel alloys are heated into a single phase region (austenite) and then cooled or quenched at various cooling rates to form multiphase structures (i.e. ferrite and cementite). Depending on how the steel is cooled, a wide variety of microstructures (ie. pearlite, bainite and martensite) can be obtained with a wide range of properties.
Another approach to steel technology is called glass devitrification, producing steels with bulk nanoscale microstructures. The supersaturated solid solution precursor material is a super cooled liquid, called a metallic glass. Upon superheating, the metallic glass precursor transforms into multiple solid phases through devitrification. The devitrified steels form specific characteristic nanoscale microstructures, analogous to those formed in conventional steel technology.
It has been known for at least 30 years, since the discovery of metallic glasses, that iron based alloys could be made into metallic glasses. However, with few exceptions, these iron based glassy alloys have had very poor glass forming ability and the amorphous state could only be produced at very high cooling rates (>106 K/s). Thus, these alloys may be processed by techniques which give very rapid cooling such as drop impact or melt-spinning techniques.
While conventional steels have critical cooling rates for forming metallic glasses in the range of 109 K/s, special iron based metallic glass forming alloys have been developed having a critical cooling rate orders of magnitude lower than conventional steels. Some special alloys have been developed that may produce metallic glasses at cooling rates in the range of 104 to 105 K/s. Furthermore, some bulk glass forming alloys have critical cooling rates in the range of 100 to 102 K/s, however these alloys may employ rare or toxic alloying elements to increase glass forming ability, such as the addition of beryllium, which is highly toxic, or gallium, which is expensive. The development of glass forming alloys which are low cost and environmentally friendly has proven much more difficult.
In addition to the difficulty in developing cost effective and environmentally friendly alloys, the very high cooling rate required to produce metallic glass has limited the manufacturing techniques that are available for producing articles from metallic glass. The limited manufacturing techniques available have in turn limited the products that may be formed from metal glasses, and the applications in which metal glasses may be used. Conventional techniques for processing steels from a molten state may provide cooling rates on the order of 10−2 to 100 K/s. Special alloys that are more susceptible to forming metallic glasses, i.e., having reduced critical cooling rates on the order of 104 to 105 K/s, may not be processed using conventional techniques with such slow cooling rates and still produce metallic glasses. Even bulk glass forming alloys having critical cooling rates in the range of 100 to 102 K/s, may be limited in the available processing techniques, and have the additional processing disadvantage in that they may not be processed in air but only under very high vacuum.